Variations in Vitrinite Reflectance and Clay-Mineral Composition Within Vertical Sections of Belden Shale, Eagle Basin, Colorado--Problems in Determining Thermal Maturity

Vito F. Nuccio, Richard M. Pollastro, Christopher J. Schenk

A major problem in defining "true" thermal maturity of shales by mean random vitrinite reflectance (R_m) and clay-mineral geothermometry is determining the correct population of organic matter and/or interstratified illite/smectite (I/S) clay, respectively. Shales commonly contain a variety of both organic matter and I/S clay; the types of both the organic matter and the I/S clays vary widely, as do the interpreted values, reflecting the heterogeneous sources. The organic-rich, carbonate-bearing Belden Shale (Early Pennsylvanian) contains three distinct populations of organic matter. The vitrinite with the lowest reflectance is weathered-looking or porous, and is generally in the 0.90-1.30% R_m range. The next higher population of vitrinite R_{m values is consistently in the 2.0-3.0% range. The highest Rm population is fusinite, measuring in the 4.0-5.0% range. Other indicators of thermal maturity (Tmax and time-temperature index reconstructions), which show that these rocks have been subjected to relatively high temperatures, are in best agreement with the intermediate Rm values (2.0-3.0%).}

Within a vertical section of the Belden Shale, randomly interstratified I/S commonly coexists with ordered and highly illitic I/S; this relationship may vary significantly from sample to sample. Although the ordered and highly illitic I/S is in good agreement with the other thermal indicators, randomly interstratified I/S typically does not exist in sedimentary rocks with R_m > 0.80%.

Possible explanations for the problems stated are (1) that weathering of precursor organic matter and minerals can produce lower values in R_m and highly expandable I/S, respectively; (2) suppression effects reported for vitrinite in other organic-rich marine shales and for the smectite-to-illite reaction in other calcium-bearing rocks; and (3) very late diagenesis at near-surface conditions producing highly expandable I/S.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.